The fiber-reinforced resin plate in which a phenolic resin is impregnated into a fibrous base material and cured to thereby form a matrix, for example, a fiber-reinforced resin laminate, has conventionally been employed in various industrial materials such as electrical insulating materials. The fiber-reinforced resin plate in which a phenolic resin is employed as a matrix may be produced by first impregnating a fibrous base material with a phenolic resin varnish obtained by dissolving a phenolic resin in an organic solvent, secondly drying the fibrous base material for half-curing the phenolic resin to thereby prepare prepregs, thirdly laminating a predetermined number of prepregs and finally completely curing the phenolic resin.
For example, a resol-type phenolic resin obtained by reacting a phenol compound such as phenol and an alkylated phenol with formaldehyde has conventionally been used as such a phenolic resin for use in the production of the fiber-reinforced resin plate.
However, the fiber-reinforced resin plate prepared with the use of the above resol-type phenolic resin has drawbacks in that not only are its electrical insulating properties low but also its heat resistance is unsatisfactory.
Fiber-reinforced resin plates respectively containing various modified phenolic resins as matrixes have been proposed in order to overcome the above drawbacks. These fiber-reinforced resin plates respectively containing modified phenolic resins include, for example, a fiber-reinforced laminate of resol-type oil-modified aromatic hydrocarbon/phenolic resin (see Japanese Patent Publication No. 51(1976)-16068), a fiber-reinforced laminate of oil-modified mesitylene/phenol/formaldehyde resin (see Japanese Patent Publication No. 53(1978)-5707) and a glass-fiber-reinforced phenolic resin plate which contains great amount of magnesium hydroxide and, as a fiber base material, a non-woven fabric of glass fiber (see Japanese Patent Laid-Open Publication No. 2(1990)-39928).
The epoxy resin has low molding shrinkage and is excellent in heat resistance, wear resistance, chemical resistance and electrical insulating properties, so that its use has been attempted as a matrix of a fiber-reinforced resin plate. However, the fiber-reinforced resin plate using the epoxy resin alone as the matrix has poor heat resistance and difficulty in retaining strength at the time of heating, so that its practical application has encountered various problems.
Thus, various fiber-reinforced resin plates have been proposed for resolving the problems, which include, for example, a fiber-reinforced epoxy resin laminate containing a novolak-type phenolic resin (see Japanese Patent Laid-Open Publication No. 2(1990)-73824), a glass-fiber-reinforced bisphenol A epoxy resin laminate containing an imidazole (see Japanese Patent Laid-Open Publication No. 64 (1989)-1755), a fiber-reinforced resin laminate containing a matrix whose mixing principal resins are an epoxy resin containing a novolak resin and a bisphenol sulfone (see Japanese Patent Laid-Open Publication No. 63(1988)-234014) and a fiber-reinforced resin laminate containing a matrix whose mixing principal resins are an epibis epoxy resin containing a novolak resin, a phenolic-novolak-type epoxy resin and bisphenol A (see Japanese Patent Laid-Open Publication No. 63(1988)-37138).
However, the above conventional fiber-reinforced resin plates and fiber-reinforced resin laminates each containing a matrix composed of a modified phenolic resin or an epoxy resin are still unsatisfactory in electrical insulation, heat resistance and strength retention at the time of heating.
The inventors previously proposed fiber-reinforced resin laminates prepared using a varnish containing a modified phenolic resin obtained by poly-condensation of a petroleum heavy oil or pitch, a formaldehyde polymer and a phenol, which resin contains substantially no acid, and an epoxy resin as a result of the development for a fiber-reinforced resin laminate having both of the electrical insulating properties, heat resistance, etc. and the mechanical strength, dimensional stability, etc. improved with a good balance (see Japanese Patent Laid-Open Publication Nos. 4(1992)-348933 and 5(1993)-16276). However, even the above fiber-reinforced resin laminates are unsatisfactory in heat resistance, dimensional stability and mechanical properties at the time of heating, so that further improvement has been desired.
Thus, in substitution for the above modified phenolic resin, the inventors proposed the use of a modified phenolic resin having a low-molecular-weight-component content of 10% by weight or below and containing substantially no acid as the matrix (see Japanese Patent Laid-Open Publication No. 5(1993)-217900).
However, the reactivity of this modified phenolic resin with an epoxy resin is not satisfactory, so that further improvements in mechanical properties such as strength and dimensional stability and heat resistance have been demanded from the fiber-reinforced resin plate containing the above resin.
Often, the fiber-reinforced resin plate has its one side surface bonded to a metal foil to use as a wiring substrate, etc. The above conventional fiber-reinforced resin plates are required to have further improvement in the adhesion to a foil of a metal such as copper.
The inventors have conducted investigations with respect to the above drawbacks of the prior art. As a result, they have found that the reactivity with an epoxy resin is surprisingly improved by lowering the molecular weight of the modified phenolic resin to be blended with the epoxy resin for forming a matrix of a fiber-reinforced resin plate through a reaction of the modified phenolic resin with a phenol in the presence of an acid catalyst, and that a fiber-reinforced resin plate which is excellent not only in adhesion to a metal such as copper, heat resistance and electrical insulating properties but also in dimensional stability, strength and other mechanical properties, especially those at the time of heating, can be provided by employing a resin composition comprising the above reactivity-improved modified phenolic resin and an epoxy resin as a matrix material. The present invention has been completed on the basis of these findings.